Although
known for quiet eruptions of lava that rarely endanger lives,
Kilauea Volcano on the Island of Hawai'i has also produced
devastating and deadly explosive eruptions in the past. Because
such eruptions are sure to occur again, U.S. Geological Survey
(USGS) scientists are studying deposits from Kilauea's past
explosive eruptions to better understand what caused them and
to identify communities at risk from such eruptions in the
future.

The late 1780's were years of great strife on the Island of Hawai'i. Kamehameha,
who later became the first king of the Hawaiian Islands, was at war with his
rival Keoua. After one of several indecisive battles, probably in 1790, the
balance was suddenly tipped in favor of Kamehameha when a natural disaster struck.
As a large group of Keoua's warriors traveling with their families passed the
crater of Kilauea Volcano, there was a sudden explosive eruption of searingly
hot ash and gas. At least 80 and perhaps hundreds of people were killed in the
deadliest historical eruption to occur in what is now the United States.

About
the year 1790 and in 1924, deadly explosive eruptions occurred at Kilauea
Volcano, Hawai'i. Similar but much larger explosive eruptions devastated
the volcano's summit area several times in the past few thousand years.
In this photograph from 1973, steaming Halema'uma'u Crater, the site of
the 1924 explosive eruption, lies within the larger volcanic depression
(caldera) at Kilauea's summit. The U.S. Geological Survey's Hawaiian Volcano
Observatory (lower center) perches on the caldera's northwest rim. Inset
(courtesy of the Bishop Museum) shows tourists posing at a safe distance
during the 1924 eruption.

Since 1790, only one much smaller explosive eruption has occurred in Hawai'i.
During 18 days in May 1924, hundreds of steam explosions from Kilauea hurled
mud, debris, and hot rocks weighing as much as 8 tons (7,000 kg) as far as two-thirds
of a mile (1 km) from the center of Halema'uma'u the current crater within the
larger volcanic depression (caldera) at Kilauea's summit. Columns of volcanic
ash and dust rose more than 2 miles (3 km) into the air, at times turning day
into night at the town of Pahala, nearly 20 miles (30 km) downwind. Fortunately,
only one person was killed during this eruption, a photographer who ventured
too close and was struck by falling rocks and hot mud.

Kilauea's
Explosive Eruptions can Produce Significant Ash Deposits

The
series of eruptions that produced the Keanakako'i Ash (top photo above
being examined by a U.S. Geological Survey scientist) devastated an area
of at least 75 square mile s (190 km 2). In about 1790, one
of these eruptions struck a large party of Hawaiian warriors, killing
at least 80 people in the deadliest historical eruption to occur in what
is now the United States. Smaller photo above (courtesy National Park
Service) shows one of hundreds of footprints, possibly made by the warriors,
preserved in the Keanakako'i Ash.

At
least two explosive eruptions between 2,700 and 2,000 years ago produced
the Uwekahuna Ash, devastating an area almost two-thirds as large as the
230-square-mile (590-km 2) "blast zone" of the May 18, 1980,
eruption of Mount St. Helens, Washington. Even older ash deposits are
visible along the Hilina Pali fault scarp.

Explosive eruptions do not generally come to mind when people think of Hawai'i's
volcanoes. Their eruptions are typically characterized by the relatively quiet
outflow of very fluid lava and by sometimes spectacular lava fountains. Hawai'i's
volcanoes have therefore become the textbook example of nonexplosive volcanism,
and the term "Hawaiian type" is used to refer to such eruptions. Eruptions at
Kilauea can often be observed safely at close range. For this reason, in 1912,
volcanologist Thomas Jaggar established the Hawaiian Volcano Observatory (HVO),
now operated by the U.S. Geological Survey (USGS) at the top of Uwekahuna Bluff
near Kilauea's summit. Jaggar even called Kilauea "the safest volcano on Earth"
because of its typically gentle activity.

Scientists now know that Kilauea in the past has produced numerous explosive
eruptions, many dwarfing even the one that occurred about 1790. Although explosive
eruptions at Kilauea are infrequent, deposits of ash discovered by USGS and
other scientists document that they actually occur about as often as explosive
eruptions from volcanoes in the Cascade Range of the Pacific Northwest, which
includes Mount St. Helens, Washington.

WHAT
CAUSES EXPLOSIVE ERUPTIONS AT KILAUEA?

Explosive
eruptions at Kilauea are thought to be caused when water comes into contact
with hot or molten rock (magma) and flashes into steam. In 1924, this
happened after the level of the lava lake in Halema'uma'u Crater dropped
below the water table. The walls of the crater then collapsed and blocked
the opening down which the lava had drained, allowing steam pressure to
build up and cause violent explosions.

Many of
Kilauea's pre-1924 explosive eruptions that produced significant ash deposits
probably happened when the volcano's summit crater was so deep that its
floor was below the water table, letting ground water seep in to form
a lake. Whenever magma erupted into the lake water, violent explosions
of steam and volcanic gases resulted, fragmenting the magma into tiny
ash particles and driving fast-moving, extremely hot ash-laden steam clouds
(pyroclastic surges) out of the crater.

At least nine layers of volcanic ash, the oldest of which are tens of thousands
of years old, are exposed along cliffs in the Hilina fault system, 6 to 8 miles
(10 to 13 km) south of Kilauea's summit. Most of these layers are at least a
few feet thick, and some are several tens of feet thick. The oldest deposits
from Kilauea's explosive eruptions that are still well enough preserved to be
mapped over a wide area can be seen at the base of Uwekahuna Bluff. Known as
the Uwekahuna Ash, these deposits represent at least two eruptions that occurred
between 2,700 and 2,000 years ago. They are more voluminous and widespread than
the younger deposits (Keanakako'i Ash) from a series of explosive eruptions
at Kilauea that ended about 200 years ago and included the eruption that decimated
Keoua's army. The Keanakako'i eruptions deposited more than a 100 million cubic
yards (75,000,000 m3) of ash, which today still blankets most of Kilauea's summit
and is in places more than 35 feet (10 m) thick.

Recent work by HVO and Smithsonian Institution scientists has discovered evidence
of an explosive eruption that occurred on Kilauea's eastern flank approximately
1,100 years ago, about 500 years after Polynesians first settled in Hawai'i.
Evidence has also been found of explosive eruptions that may have happened about
600 years ago, and ash deposits from still-younger eruptions have been found
covering sites on Kilauea used by ancient Hawaiians. How these recently discovered
eruptions affected ancient Hawaiians has yet to be determined.

Explosive eruptions at Kilauea are thought to be caused when water comes into
contact with hot rock or magma and flashes into steam. In 1924, this happened
after the lava lake in Halema'uma'u Crater sank hundreds of feet below the crater's
rim over a period of a month, draining out of sight by late February of that
year. Two months later, Halema'uma'u's floor began to sink. This weakened the
crater's walls, causing rock falls that enlarged the crater. By May 10th, large
parts of the walls had collapsed, filling and blocking the opening of the conduit
down which the lava had drained. Ground water seeping into the conduit flashed
into steam when it encountered hot rock and rapidly built up pressure beneath
the blockage. The steam then escaped violently (as in a boiler explosion), blasting
away the blockage and scattering rocks and dust into the air. During the next
17 days, this cycle of collapse, blockage, and explosion was repeated hundreds
of times.

Many of Kilauea's pre-1924 explosive eruptions that produced significant ash
deposits probably happened when the volcano's summit crater was so deep that
its floor was below the water table, letting ground water seep in to form a
lake. Whenever magma erupted into the lake water, violent explosions of steam
and volcanic gases resulted. These explosions fragmented the magma into tiny
ash particles and drove ash-laden steam clouds (pyroclastic surges), like those
that killed Keoua's warriors, out of the crater at speeds as great as 100 miles
an hour (160 km/h).

Because the water table at Kilauea currently lies about 1,700 feet (520 m)
below the rim of Halema'uma'u, there is little chance of imminent explosive
eruptions. However, since the early 1820's, the floor of Kilauea's summit crater
has dropped to within 300 feet (90 m) of the water table at least three times,
and any future subsidence of this magnitude would be cause for concern. Future
explosive eruptions of Kilauea could endanger the lives of the thousands of
people who live, work, or spend time as visitors near the volcano.

The explosive history of Kilauea continues to be revealed through investigations
by USGS and other scientists. Knowledge gained from these studies is being used
at HVO to identify communities at risk from eruptions of Hawai'i's active volcanoes.
These volcanoes are being closely monitored by HVO scientists so that they can
alert the public and local government officials to impending eruptions. The
work of HVO is part of the USGS Volcano Hazard Program's ongoing efforts to
help protect people's lives and property from volcano hazards in all of the
volcanic regions of the United States, including Hawai'i, Alaska, Wyoming, California,
and the Pacific Northwest.